The present invention pertains to medical nuclear magnetic resonance scanners, and, more particularly, to the structure employed in connection with achieving a desired static and dynamically uniform magnetic field in the patient scanning area thereof.
Since the invention of the medical nuclear magnetic resonance (NMR) scanning technique by Dr. Raymond Damadian, as set forth in U.S. Pat. No. 3,789,832, this technique has been widely adopted in the medical arts. Medical NMR scanning requires creation of a substantial constant "primary" magnetic field passing through the patient's body. Additional "gradient" magnetic fields varying with time typically are superimposed on the primary field. The patient is exposed to radio frequency electromagnetic waves which also vary with time in particular patterns. Under the influence of the magnetic fields and the radio waves, certain atomic nuclei within the patient's tissues resonate and emit further radio waves. By known mathematical techniques involving correlation of the magnetic field patterns in use at various times with the radio frequency waves emitted, it is possible to determine the amount and/or physical state of particular atomic nuclei, or a physical condition at various locations within the patient's body. This information typically is displayed as an image with shadings corresponding to the concentration and/or physical state of certain nuclei of interest. Alternatively, it can be displayed as spectral information. The concentrations or physical state of different substances ordinarily differ for differing kinds of tissues. Thus, the image created by NMR techniques permits the physician to see organs and soft tissues within the body, and also permits the physician to see abnormalities, such as tumors, within the body. Accordingly, NMR scanning and imaging techniques are being adopted rapidly by physicians.
Medical NMR scanning imposes certain challenging technical requirements for the apparatus. The primary magnetic field must be a strong field, typically from less than 1 kilogauss to more than 10 kilogauss (1 Tesla). This is far stronger than the magnetic fields associated with many common magnets. Moreover, the primary magnetic field must be precisely configured. Thus, the primary field, before application of the gradient fields, should be uniform to at least about 1 part in 1,000 and preferably at least about 1 part in 10,000 or better, over the imaging volume, in order to provide a useful image. Even better uniformity is more desirable. This strong uniform primary magnetic field must be maintained over a region of the patient's body large enough to provide medically useful information, typically over a scanning region encompassing a major portion of a cross section through the patient's torso. Further, the magnetic field apparatus typically must be arranged to receive the patient's body, and hence must provide openings large enough for the patient's body to fit within the apparatus. All these requirements, taken together, pose a formidable technical problem.
Two distinct and fundamentally different approaches to these requirements are currently employed in construction of medical NMR scanners. As set forth in commonly assigned U.S. Pat. No. 4,675,609 to Danby et al., magnetic field producing means such as permanent magnets or excitation coils can be combined with a ferromagnetic metal frame and other components to form a magnetic assembly which provides the primary field. The disclosure of said U.S. Pat. No. 4,675,609 is hereby incorporated by reference herein. Medical NMR scanners incorporating magnetic assemblies according to U.S. Pat. No. 4,675,609 have excellent primary fields and hence offer good scanning capabilities.
The other approach has been to employ essentially solenoidal electromagnets having resistive or superconducting windings. The windings of a superconducting magnet under appropriate temperature conditions lose resistance to flow of electric current. Thus, superconducting magnets can carry large currents and can create high fields. Some superconducting electromagnets have been built as essentially air core solenoids, with only minor ferromagnetic elements. Alternatively, as set forth in commonly assigned U.S. Pat. No. 4,766,378 to Danby et al., superior superconducting magnets for NMR scanners can be made using a ferromagnetic frame to direct and shape the flux into the patient-receiving space and to provide a flux return path.
As indicated earlier, and as may be seen in the aforesaid commonly assigned U.S. Pat. No. 4,766,378, gradient coils are used to superimpose gradient magnetic fields on the primary field generated by the primary field generating apparatus, such as primary windings or permanent magnets. The gradient coils are disposed within the frame, adjacent the patient-receiving space. The available space within the frame is limited, in that large openings within the frame tend to reduce the strength of the primary field. To provide a patient-receiving space of adequate size, the gradient coils ordinarily are disposed in proximity to the ferromagnetic materials of the primary magnetic field assembly. It is typically desirable in NMR scanning to vary the gradient fields imposed by the gradient coils at relatively rapid rates. This results in the creation of eddy currents in the ferromagnetic materials, which in turn cause undesirable non-uniformities in the magnetic field. The aforementioned '378 patent discloses certain magnet configurations which provide enhanced clearance within the frame, and hence allow increased distance between the gradient coils and the ferromagnetic materials.
Despite these improvements there has been a significant need heretofore for further improvement with respect to increasing the uniformity of the main magnetic fields of medical NMR scanners by reducing the non-uniformities introduced therein due to gradient-coil-induced eddy currents. Moreover, some versions of the scanners disclosed in the '378 and '609 patents have limited clearances therein and thus also have need of the present invention.